This invention relates generally to incandescent lamps and, more particularly, to incandescent lamps configured to provide improved energy efficiency and to methods for making such lamps. This invention also relates generally to incandescent illumination systems for projecting a beam of light and, more particularly, to incandescent illumination systems of a kind that reflect IR light back to an incandescent lamp's filament, to increase the system's energy efficiency.
Prior incandescent lamps typically have included one or more filaments supported at their ends by a bridge assembly containing components formed of tungsten and quartz. Although most of the light emitted by the filament(s) is emitted outwardly from the lamp, a portion of it is emitted in directions toward the lamp's base end or toward the tungsten/quartz bridge assembly, where it is generally wasted, either by absorption or by scattering in undesired directions.
In addition, prior incandescent illumination systems of this kind typically have included a lighting fixture that mounts an incandescent lamp with its filament(s) located at or near the focal point of a concave reflector. Light emitted by the lamp is reflected by the reflector, to project a beam of light. In some cases, the incandescent lamp has included an IR-reflective coating in the form of a multi-layer stack of dielectric material coated directly onto the lamp's envelope. The coating functions to transmit visible light but reflect infrared light back to the lamp filament, where a portion of that reflected light is absorbed. This absorption heats the filament and thus reduces the amount of electrical energy required to heat the filament to its operating temperature. This improves the lamp's energy efficiency. The system typically is embodied in a wash-light fixture, for projecting a non-imaged beam of light, but alternatively could be embodied in an imaging lighting fixture, for projecting an image at a distant location.
Incandescent illumination systems of this kind are not believed to have been as energy-efficient or cost-effective as possible. One drawback has arisen because the IR-reflective coating typically has been located on the lamp envelope itself, which requires that the coating be replaced whenever the lamp burns out or otherwise fails. The coating can represent a significant portion of the lamp's manufacturing cost, so this requirement has raised the system's overall operating cost. Another drawback is that the IR-reflective coatings have not reflected as much IR light as is possible, while remaining cost-effective. One example of such an incandescent lamp is disclosed in U.S. Pat. No. 4,017,758 to Almer et al.
Yet another drawback to the incandescent illumination systems of this kind is that the systems have failed to collect a significant amount of light emitted by the lamp filament(s) in directions other than directly toward the concave reflector, i.e., light emitted in a forward direction beyond the reflector's forward extent or in a rearward direction toward the lamp's base. This light fails to strike the concave reflector and is either absorbed by the system or projected as stray light outside the projected beam's desired field angle. The absorption by the system causes excessive heating, which generally has required the system to comprise a housing made of metal, thus adding undesired weight and cost. In addition, the stray light is highly undesirable when the system is intended to illuminate only specific areas or objects.
One attempt to design an incandescent lamp that better utilizes light emitted by the lamp filament in undesired directions, e.g., in a direction toward the lamp's base, is disclosed in the Almer et al. patent, identified above. The disclosed lamp includes concentric, cylindrical inner and outer envelopes, with a filament extending longitudinally within the inner envelope. Two reflective, disc-shaped filament supports are located at the opposite ends of the inner envelope and two reflective rings are located at the opposite ends of the space between the two concentric envelopes, in alignment with the disc-shaped filament supports. An IR-reflective coating, incorporating both an interference filter and a metal oxide filter, is located on the inner surface of the outer envelope. This coating is configured to reflect infrared light back toward the filament and transmit visible light outwardly.
One lamp disclosed in the Almer et al. patent is said to provide a very high efficiency of 44.9 lumens per watt, nearly double the efficiency of a similar lamp lacking an IR-reflective coating. It is apparent, however, that any such high efficiency would have been short-lived, making the lamp of limited commercial value. This is because the metal oxide filter likely would have been rapidly degraded by the infusion of oxygen from the adjacent interference filter or outer envelope. The patent lacks any suggestion of a solution to this degradation problem; in fact, it lacks even a recognition of the problem itself. The patent also lacks any disclosure of suitable materials for its reflective disc-shaped filament supports and its reflective rings. These deficiencies might explain the lack of any apparent commercialization of the lamp, despite its stated improvement in efficiency.
It should, therefore, be appreciated that there remains a need for an improved incandescent lamp, and for an improved incandescent illumination system, that are configured to more completely collect and utilize light emitted by the lamp filament(s). It should also be appreciated that there remains a need for an improved incandescent illumination system configured to avoid the need to replace an IR-reflective coating when the system's incandescent lamp is replaced. The present invention satisfies these and other needs.